# Change DC voltage?

#### jhelmick

Joined Mar 24, 2006
4
This simple task is beyond me, but I have a Tach Generator that outputs 110 VDC at its maximum RPM, I need that DC voltage to be 10 volts maximum for an input signal to a inverter drive motor controller.
Here is my problem...I was told 1000 ohms will reduce the voltage by 1, therefore 100K ohms would reduce the voltage by 100, not according to my meter...I tried that and it reduced the voltage maybe 1/2 volt. what am I to do

#### Papabravo

Joined Feb 24, 2006
19,244
Originally posted by jhelmick@Mar 24 2006, 08:36 PM
This simple task is beyond me, but I have a Tach Generator that outputs 110 VDC at its maximum RPM, I need that DC voltage to be 10 volts maximum for an input signal to a inverter drive motor controller.
Here is my problem...I was told 1000 ohms will reduce the voltage by 1, therefore 100K ohms would reduce the voltage by 100, not according to my meter...I tried that and it reduced the voltage maybe 1/2 volt. what am I to do
[post=15399]Quoted post[/post]​
A single resistor will not do diddley be-bop for you. What you need is a voltage divider. That's two resistors, call them R1 and R2. What you want is to multiply 110 VDC by 0.090909...

Rich (BB code):
Vout = Vin(R2/(R1+R2)) = 110*(10K/(100K + 10K)) = 10
Connect a 100K resistor and a 10K resistor together. Connect the free end of the 100K to your input and connect the free end of the 10K to ground. If you measure the voltage across the 10K resistor with respect to ground you should measure 0.090909... times the input.

You should stop listening to your little friends with the quaint notions.